The invention relates to a method for operating a fuel system of a combustion engine, wherein the fuel system has a fuel tank and a ventilation device which has at least one ventilation valve for ventilating the fuel tank in the direction of a device which at least temporarily generates negative pressure. The invention also relates to a fuel system.
Methods of the aforementioned type are known from the state-of-the-art. The corresponding fuel system is for example assigned to a motor vehicle or a drive system of the motor vehicle. The drive system has in particular at least one combustion engine and is for example configured as hybrid drive system, i.e., it includes the combustion engine as well as at least one electric machine, wherein the combustion engine and the electric machine generate a drive torque of the drive system at least temporarily in cooperation. The fuel system supplies fuel from the fuel tank to the combustion engine. Oftentimes a volatile hydrocarbon fuel is used as fuel, for example gasoline. The fuel tank therefore normally contains a volume of liquid fuel as well as a volume of gaseous fuel, which in particular accrues above the liquid fuel. The fuel tank can be a closed tank, in particular a pressure tank or a partially closed in particular also pressure-less tank. The closed tank is particularly used for reducing emissions.
Fluctuations of the temperature of the fuel, for example caused by changes of the ambient temperature, can cause pressure fluctuations in the fuel tank. For this reason a ventilation device is assigned to the fuel tank. It serves for ventilating the fuel tank, which allows reducing excessive pressure in the fuel tank. For this purpose the ventilation device ventilates the fuel tank for example through a ventilation line. During ventilation, gaseous and liquid fuel can exit the fuel tank through the ventilation device or the ventilation line. The ventilated fuel thus is first present as a mixture of gaseous and liquid fuel. This is in particular the case when the fuel tank is ventilated at high pressure inside the fuel tank. The high pressure or the high pressure differential between the pressure inside the fuel tank and the pressure outside the fuel tank causes high flow velocities of the ventilated fuel which causes liquid fuel to be carried along by the gaseous fuel.
The gaseous fuel can be supplied to the combustion engine or its intake system, wherein a fuel accumulator, which is preferably configured as activated carbon accumulator, can be arranged between the fuel tank and the combustion engine. The fuel accumulator serves for temporarily storing gaseous fuel, i.e., to take up gaseous fuel when unneeded gaseous fuel is present and to give off gaseous fuel as soon as the gaseous fuel can be supplied into the combustion engine. However, no liquid fuel must enter the fuel accumulator or the combustion engine in order to prevent damage or malfunction.
For this reason, the ventilation device can have at least one separation device, which serves for separating gaseous and liquid fuel. The separation device is thus intended to prevent transfer of liquid fuel from the fuel tank through the ventilation device into the combustion engine or the fuel accumulator. The separation device separates liquid fuel and allows gaseous fuel to pass. The separated liquid fuel enters a temporary accumulator of the separation device. The term temporary accumulator does not mean that an actual (temporary) storing of the liquid fuel is intended. Rather, the liquid fuel can be directly discharged from the temporary accumulator or the separation device, preferably in the direction of the fuel tank. During the course of this, however, the fill level of the temporary accumulator may increase, for example due the discharge volume flow being limited, in particular by a line cross-section or the like. The separated liquid fuel can thus at least temporarily not be discharged as fast as it is introduced into the temporary accumulator. However, a temporary storing of the liquid fuel for example over a defined period of time can be realized.
During operation of the fuel system it should be prevented that the amount of liquid fuel in the temporary accumulator or the separation device exceeds a threshold amount, i.e., that the fill level of the temporary accumulator becomes greater than a threshold fill level, since this can adversely affect the efficiency of the separation device. The greater the amount of liquid fuel in the temporary accumulator the greater the risk that liquid fuel also exits the separation device together with the gaseous fuel and is carried along in the direction of the fuel accumulator or the combustion engine. For this reason, the fuel delivery device can be assigned to the separation device. The fuel delivery device is used to transport liquid fuel from the temporary accumulator, in particular in the direction of the fuel tank. The fuel delivery device is usually configured as suction jet pump, wherein fuel is often used as operating medium of the suction jet pump and is delivered by a fuel pump of the fuel system out of the fuel tank in the direction of the combustion engine.
In known methods for operating the fuel system of the combustion engine, the fuel tank is ventilated by means of the ventilation device, usually based on the internal pressure of the fuel tank and a temperature. This means that for ventilating the fuel tank the ventilation valve of the ventilation device is adjusted based on the internal pressure in the fuel tank and the temperature. However, this is only possible when the combustion engine is activated because only in this case a control device is activated, which is assigned to the combustion engine or the fuel system. This control device serves for adjusting the ventilation valve based on the, in particular measured, internal pressure of the fuel tank and the, in particular measured, temperature for ventilating the fuel tank. In phases of standstill, i.e., when the combustion engine is deactivated and with this the control device is deactivated, the internal pressure of the fuel tank is usually limited by at least one mechanical overflow valve, which is set to a typical pressure level. When the temperature in the fuel tank changes, for example due to heat introduction by the combustion engine, which is still hot from operation, or due to external influences, a pressure is established in the fuel tank which corresponds to the vapor pressure of the fuel. When this internal pressure of the fuel tank reaches or exceeds the typical pressure level and in particular a defined maximal internal pressure of the fuel tank, the overflow valve opens for ventilating the fuel tank. A deactivated combustion engine is to be understood as a combustion engine that stands still, while an activated combustion engine is at least operated in neutral and in particular provides a torque.
When the combustion engine is deactivated, the fuel tank is thus usually ventilated exclusively due to the internal pressure of the fuel tank. However, it is often the case that the admissible maximum internal pressure of the fuel tank changes with the temperature, i.e., it is dependent on the temperature. In particular when the combustion engine is deactivated and the internal pressure of the fuel tank can only be lowered via the overflow valve, the fuel tank may thus be impinged with an internal pressure, which results in forces that exceed the strength of the fuel tank. This results for example in unacceptable flow behaviors or in undesired, irreversible deformations of the fuel tank, in particular of a fuel tank shell.
The fuel accumulator, which is provided for the fuel system, has to be regenerated from time to time. This occurs for example by scavenging the fuel accumulator or increasing the cross-section. During scavenging, scavenging air is pumped through the fuel accumulator, preferably in the direction of the combustion engine or its intake system. However, scavenging of the fuel accumulator or increasing the cross-section causes a negative pressure. The fuel accumulator is thus at least temporarily a device that generates negative pressure. In particular, the fuel accumulator is a regenerable filter device. Because the fuel accumulator is connected with the fuel tank via the ventilation device, a negative pressure may also develop in the fuel tank, i.e., the pressure inside the fuel tank is smaller than the outside pressure. Herein, the deformation of the fuel tank can occur analogous to the aforementioned case of overpressure.